CALIBRATION
Calibration is an indispensable procedure for defining the metrological characteristics of a measuring instrument by comparing measurements with a reference instrument, called “reference”.
The definition of calibration is set out in the VIM International Vocabulary of Metrology:
“operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication”.
This operation must be performed under specified conditions and in accordance with procedures designed to achieve the specified purpose, preferably based on international standards.
As stated by ACCREDIA:
“Calibrations allow the indication error of instruments and measuring systems as well as the values of the measured samples to be determined to ensure more accurate and reliable measurement results”.
The calibration process of instruments and working standards by laboratories accredited according to ISO/IEC 17025 consists of comparison with a higher level reference instrument and thus allows the measurements they produce to acquire the fundamental property of metrological traceability to national or international standards.
Calibration should not be confused with the term ‘adjustment’ since calibration only allows the definition of the metrological characteristics of an instrument, while adjustment aims at improving its measurement accuracy, i.e. the difference between the value read by the instrument and the known value of the reference standard.
Calibration purposes
Calibration is used to determine the metrological characteristics of the instrument (accuracy, repeatability, reproducibility, hysteresis, indication or interpolation errors, measurement uncertainty, etc.) necessary to define its functionality or to verify its compliance with requirements.
Depending on the calibration results, it is possible, as provided for by the standards, to define the class to which the instrument belongs, based on the satisfaction of certain parameters established for the metrological characteristics listed above.
Calibration frequency
There is no specific rule determining the calibration frequency of a measuring instrument but advices can be given to establish this frequency, depending on estimated variables that allow the instrument to remain in the assigned metrological capacities.
Variables of the type: frequency of use, use methods and environmental conditions, improper use, doubts about measurement results, after any corrective maintenance (ref. standard UNI 10127-2).
Standards in the torque measurement field set the duration of a calibration certificate at 26 months, but it is generally suggested that this should be done at least annually, especially if there are special traceability requirements or if required by quality assurance regulations.
Metrological confirmation
Metrological confirmation means ‘the set of operations required for ensuring that measuring equipment meets the requirements for its intended use’ (standard UNI EN 10012:2004).
Metrological confirmation generally provides:
– Calibration, including adjustment
– Comparison of calibration results with the metrological requirements for the use of the equipment
– Definition of the period of validity, sealing and labelling.
Calibration must be performed by an EA accredited laboratory.
If the calibration is in accordance with the specifications, the instrument can be metrologically confirmed.
The confirmation interval, indicated on an indelible label, together with the identification of the certification details, must be defined.
A flow chart of the metrological confirmation process can be as follows
Reference standards
Il nostro Laboratorio di Taratura, Centro ACCREDIA LAT 267, ha definito procedure di taratura approvate Accredia in funzione delle diverse tipologie di strumenti per la misura di coppia che fanno riferimento a norme internazionali:
1. Standard DIN 51309: Static calibration of torque measurement devices
This is the specific standard for the calibration of torque transducers for static use, both for general use and for reference standards.
2. Directive DAkkS-DKD-R 3-7: Static calibration of indicating torque wrenches.
This standard is used for the calibration of high-precision torque wrenches and for the calibration of torque wrenches used as transfer samples for calibrating devices that are used for calibrating torque wrenches.
3. Directive DAkkS-DKD-R 3-8: Static calibration of calibration devices for torque wrenches and screwdrivers.
Used for static calibration, using transfer wrenches, of torque transducers used to calibrate torque wrenches and screwdrivers.
4. Standard ISO 6789: Hand torque tools – Requirement and test methods for design conformance testing, quality conformance testing and recalibration procedure.
This standard, which has been recently updated in its 2017 version, is the specific standard for hand-held torque wrenches, both indicating torque tools and setting torque tools (direct reading torque wrenches, snap wrenches, direct read or pre-set torque wrenches, etc.).
Calibration procedures of SCS Calibration Centre
Procedure 001: Calibration of torque transducers according to DIN 51309 standard
In particular, the procedure concerns torque transducers in general, torque transducers where the elastic deformation of a mechanical element subjected to torsion is measured by strain gauges and converted into a proportional electrical signal displayed on an indicator instrument.
Calibration is performed by comparison using a reference torque transducer calibrated by an EA calibration Centre, whose extended uncertainty, including all influences, is less than that indicated in our accreditation table (<0.2%)
In our laboratory, we offer several calibration benches, each equipped with reference torque transducers of various capacities, which offer the possibility of calibrating torque transducers in the range 0.2 – 5000 N·m
The following photos are examples of torque transducer calibrations on different calibration benches.
Procedure 002: Calibration of torque transducer with transfer wrench according to DKD-R 3-8 directive
The procedure concerns the calibration of torque transducers used in torque wrench and screwdriver calibration systems using a so-called “transfer torque wrench”.
Transfer torque wrenches are particularly insensitive to side or longitudinal forces and bending moments: they have a “head”, consisting of a torque transducer with special characteristics, for transmitting the torque by means of a square or other coupling, to be connected to the torque transducer during calibration..
The purpose of this calibration method is to apply to the torque transducer under calibration the components of side forces and bending moments simulating the behavior of the torque tools intended to be calibrated by means of these torque transducers.
Calibration is performed by comparison using a reference transfer wrench calibrated by an EA calibration Centre according to DKD-R 3-7, whose extended uncertainty, including all influences, is less than that indicated in our accreditation table (<0.2%)
In our laboratory, we offer various transfer wrenches of various capacities that offer the possibility of calibrating torque transducers in the range 0.2 – 5000 N·m
The following photos are examples of transfer wrench and example of calibration.
Procedure 003: Calibration of torque wrenches according to DKD-R 3-7 directive
The procedure concerns the calibration of high-precision torque wrenches and “transfer” torque wrenches, instruments where the elastic deformation of a mechanical element (torsion or bending) is measured by strain gauges and converted into a proportional electrical signal displayed on an indicating instrument, placed on board of the wrench or externally connected by cable or other means of signal transmission.
Calibration is performed by comparison using a reference torque transducer calibrated by an EA calibration Centre, whose extended uncertainty, including all influences, is less than that indicated in our accreditation table (<0.2%).
The special feature of the calibration of these reference torque transducers is that the transducer is calibrated according to DKD-R 3-8 using transfer torque wrenches: this is to simulate the components of side forces and bending moments on the sample that the torque transducer will undergo during the calibration for which it is intended.
In our laboratory, we offer several reference torque transducers of various capacities that offer the possibility to calibrate torque wrenches in the range 0.2 – 5000 N-m
The following photos are examples of calibrations of torque wrenches on different calibration benches.
Procedure 004: Calibration of torque wrenches according to ISO 6789 standard
The procedure concerns the calibration of generic torque wrenches and is intended to verify if are met the requirements of ISO 6789:04, now replaced by an updated version ISO 6789:17 Part 1.
The calibration procedure is similar to Procedure 003: the same reference samples are used but with a different sequence of the calibration cycle.
If the deviations of measures detected on the torque wrench in calibration are within 4% of the reference, the instrument shall comply with ISO 6789.
More information on calibration standards and examples of certificates according to standards
The following links allow further detailed analysis of the calibration standards and access to examples of calibration certificates drawn up in accordance with the calibration standards and procedures described above.
